The invention relates to a method and device for converting a preferably weakly acid ion exchanger material from the H form into the Ca form.
Weakly acid ion exchanger materials in the Ca form are in a position to precipitate lime from calcareous waters by the catalytic route. For this catalytic effect to clearly be of benefit, it is necessary to convert the ion exchanger material as fully as possible into the Ca form. A residual loading of the material with H+ ions (H form) would reduce the pH value of the water in use with water as a result of occurring ion exchange and in this way counter the catalytic action of the functional groups loaded with Ca2+ ions.
In order to achieve this, the invention proposes in a first variant that the ion exchanger material is brought into contact with an aqueous, preferably saturated, calcium hydroxide Ca(OH)2 solution. The corresponding device is characterized by a conversion tank with a water inlet, a filling opening for the ion exchanger material and a filling opening for calcium hydroxide Ca(OH)2 or calcium oxide CaO, plus an extraction opening (FIG. 1).
Customary ion exchanger materials available in the trade in the H form (e.g. Lewatit S 8528 from Bayer A G, Leverkusen, Germany) have the property that an ion exchange with other cations becomes possible only from a specific pH value; this pH value threshold lies, depending on the ion exchanger material, at pH values between 3 and 6 (depending also on the opposed ion involved and its concentration). For this reason, there is a limit to a direct conversion of the materials, for example in a CaCl2 solution (calcium chloride solution): the ion exchange stops as soon as the pH value threshold is not reached. Measures are therefore necessary which make the ion exchange possible at high pH values.
With strongly acid ion exchangers, the ion exchange is less sensitive to pH values. However, with these resins also, the ion exchange practically comes to a halt at very low pH values ( less than 1).
Ca(OH)2 has a relatively low solubility in water. At T=20xc2x0 C. it is 1.7 g/l or 23 mmol/l; the pH value is ca. 12.3 pH units. Initially, this prevents the person skilled in the art from considering Ca(OH)2 as a suitable conversion agent. But if, for example, an aqueous solution is in contact with an adequate sedimentary deposit of Ca(OH)2, some of this sedimentary deposit goes into solution whenever Ca2+ ions or OHxe2x88x92 ions are consumed by any processes in the solution and the solubility product is not reached. This post-dissolution of Ca2+ or OHxe2x88x92 ions proceeds particularly quickly when the sedimentary deposit is whirled up by stirring and Ca(OH)2 colloids form.
If a weakly or strongly acid ion exchanger material in the Hxe2x88x92 form is added to a suspension of the latter kind, there is an immediate ion exchange of Hxe2x88x92 ions from the ion exchanger for Ca2+ ions from the solution. The Hxe2x88x92 ions passing into the solution are immediately neutralized by the OHxe2x88x92 ions present in the solution (H++OHxe2x88x92xe2x86x92H2O). The Ca2+ ions and OHxe2x88x92 ions now missing from the solution are replaced by post-dissolution from the sedimentary deposit and the Ca(OH)2 suspension respectively.
The use of a saturated Ca(OH)2 solution has the following advantages:
Ca(OH)2 is a cheap chemical which can be easily handled without special safety equipment.
The conversion needs only as much Ca(OH)2 as is required by stoichiometry. In the case of conversion of the weakly acid ion exchanger Lewatit S 8528 from Bayer with a capacity of 4.3 val/l an introduction of 2.37 mol Ca(OH)2 per litre resin (10% excess) permits a satisfactory conversion of the resin.
As an alternative to Ca(OH)2 as basic chemical, calcium oxide CaO can also be used: CaO reacts with water to give Ca(OH)2.
CaO+H2Oxe2x86x92Ca(OH)2
After the conversion, the converted ion exchanger material is rinsed out with water until the pH value 9 is clearly not reached in the draining water.
The conversion time and the rinsing-out time can be clearly reduced by raising the temperature, preferably to above 40xc2x0 C. (in particular to T=50xc2x0 C.).
An alternative possibility for the conversion of a weakly acid ion exchanger material into the Ca form is to proceed via the Na form.
A weakly acid ion exchanger material in the H form can easily be converted into the Na form with concentrated NaOH lye (caustic soda solution); for example, the weakly acid ion exchanger resin Lewatit S 8528 from Bayer can be converted by recycle pumping of a quantity of a 5-6 molar NaOH lye, equivalent to the resin bed quantity, in an exchanger column.
After the resin has been washed out with water, the resin present in the Na form can be now converted in to the Ca form for example by rinsing through three times the resin bed quantity of a 4 molar (calcium chloride) CaCl2 solution in an exchanger column (conversion tank).